...and Thorne's sell the Sublimox for £360...
Sublimox is in a different league to the Varrox (and so it should be at that price) . It is more of an active rather than passive sytem. The Varrox heats up the oxalic acid until it is at roughly the right temperature then most of it wafts its way through the brood. With the Sublimox the pan is heated to precisely the right temperature ( using a proportional–integral–derivative controller ), you then invert it, the oxalic acid immediatley sublimates and is shot out into the hive. Temperature control is rather important, otherwise the oxalic acid breaks down into formic acid and carbon dioxide, the former risks killing the queen.
I made myself a Varrox lookalike for under ten quid using a PTC heating element off ebay which was set at 170C - the right(ish) temperature for the sublimation of oxalic acid. The Sublimox is far, far preferable.
But listen to the expert, not to me:
http://theapiarist.org/sublimox/